Musannaf Ibn Abi Shayba 13:1871

Abu Bakr narrated to us, he said: Abbad bin al-Awwam narrated to us, from Husayn, who said: Umar bin Abd al-Aziz wrote: "A judge must not sit in the mosque, for the polytheists enter upon him therein, and they are indeed impure. Allah the Exalted said: 'The polytheists are only impure.'"